Look up the beep codes for your motherboard in the manual (find on their website) to confirm what the beeps mean. (Good to have downloaded/bookmarked anyway)
Next/first, with the computer turned off at the PSU (but still plugged in) try removing and re-slotting the GPU (there may be a button/leaver to release it on the motherboard. There may also be a screw holding it into the case on the inside at the back). If it only sometimes doesn’t work, that indicates it’s probably just a bad connection.